Packing for vapour-liquid contacting systems



NOV. 28, 1961 J. McwlLLlAMs 3,010,706

PACKING FOR VAPOUR-LIQUID CONTACTING SYSTEMS Original Filed Nov. 25,1955 7 Sheets-Sheet 1 INVENTOR JOSEPH ANTHONY McWlL LlAMS' Nov. 28, 1961J. A. MCWILLIAMS 3,010,706

PACKING FOR VAPOUR-LIQUID CONTACTING SYSTEMS Original Filed NOV. 25.1955 7 Sheets-Sheet 2 INVENTOR JOESPH ANTHONY MCWILLIAMS PACKING FORVAPOUR-LIQUTD CONTACTING SYSTEMS Original Filed Nov. 25. 1953 Nov. 28,1961 J, A. MCWILLIAMS 7 Sheets-Sheet 3 INVENTOR I JOSEPH ANTHONYMCWILLIAMS awe- #4 PACKING FOR VAPOUR-LIQUID CONTACTING SYSTEMS OriginalFiled Nov. 25. 1953 Nov. 28, 1961 J. A. MCWILLIAMS 7 Sheets-Sheet 4 H6.IO.

INVENTOR I JOSEPH ANTHONY MCWILLIAMS PACKING FOR VAPOUR-LIQUIDCONTACTING SYSTEMS Original Filed Nov. 25, 1955 Nov. 28, 1961 J. A.MGWILLIAMS 7 Sheets-Sheet 5 Hall.

INVENTOR JOSEPH ANTHONYMCWILLIAMS Nov. 28, 1961 0 J A, MCWILLIAMS3,010,706

I PACKING FOR VAPOUR-LIQUID CONTACTING SYSTEMS Original Filed Nov. 25.1955 7 Sheets-Sheet 6 b I I F J E 430 E INVENTOR JOSEPH ANTHONYMcWlLLlAMS Nov. 28, 1961 Original Filed Nov. 25, 1953 J. A. MCWILLIAMS3,010,706

PACKING FOR VAPOUR-LIQUID CONTACTING SYSTEMS 7 Sheets-Sheet 7 FIG. B.

My 0% 54 pm. m 5| per fool? 52 v PPEE'SUPE Fl G I 4 map [/2752 W63 5|per foot) 5 4 4m 4014 [ft/sec. l l /0 /f 20 Al? HOW ffi/sec.) l l I I I/0 l2 l3 /4 INVENTOR JOSEPH ANTHONY MCWILLIAMS United States Patent Gilice 3,010,70ii Patented Nov. 28, 1961 3,010,706 PACKING FORVAPOUR-LIQUID CONTACTING SYSTEMS Joseph Anthony McWilliams,Kidderminster, England, assignor to United Kingdom Atomic EnergyAuthority, London, England Continuation of application Ser. No. 394,455,Nov. 25, 1953. This application Apr. 13, 1959, Ser. No. 805,959 Claimspriority, application Great Britain Nov. 29, 1952 3 Claims. (Cl. 261100)This invention rel-ates to packings for vapour-liquid and gas-liquidcounter-current contacting systems, such as distillation columns,adsorption columns and cooling towers, made from foraminous materialshaped to provide a cellular structure. The present application is acontinuation of application Serial No. 394,455, filed November 25, 1953, now abandoned.

A selection of the literature referring to the abovementioned type ofpackings includes the Stedman packing (US. Patent No. 2,047,444), theWatson packing (Ind. Chem. 1949, 25,503) and the Scofield packing (US.Patent No. 2,470,652). The Stedman packing comprises a column ofcapillary material, the capillary openings of which seal with liquidunder treatment, arranged to provide a multiplicity of cells uniformlydistributed throughout the column, vapour passages through saidcapillary material larger than said capillary openings whereby each cellreceives vapour from at least two other cells and uniformly disposedcontact surfaces throughout the column to provide for repeatedsubdivision and recombination of liquid passing through the column. Inthis packing the liquid phase remains on the walls of the cells and thevapour phase passes through the vapour passages in the walls which arelarge enough not to be filmed over by the liquid phase so that only thevapour phase exists in the cells. Mesh openings of .01 inch are quotedas being satisfactory.

The Watson packing followed the principles of the Stedman packing butuses a double sheet of gauze. The packings were made from 36 mesh gauzewith a wire diameter of 0.01 in.

The Scofield packing comprises a plurality of superposed porous mats ofundulating shape, the ridges of each mat contacting the troughs of themat above it to provide for downward flow of liquid through the packing,each mat comprising a plurality of undulating foraminous metal sheets incapillary contact, the openings in the sheets being larger thancapillary size and in non-registering relation, thereby forming tortuousvapour passages through the mats unsealed by the liquid. In thisScofield packing a substantial portion of the liquid phase is conducteddownwardly along the individual filaments or strands around the openingsin the sheet and it is essential that the sheets in the mats have anon-filming characteric so that voids exist to create the tortuousvapour passages. Scofield points out that a single sheet of coarsemeshed screen is incapable of controlling or directing the how of largevolumes of liquid along its inclined surface and consequently a numberof such superimposed sheets are required.

It is an object of the present invention to provide a packing forvapour-liquid and gas-liquid contacting systems which is relativelycheap to construct so that the use of stainless steel for theconstruction of the packing becomes economically feasible.

It is a further object of the invention to provide a packing suited forhigh throughput with small pressure drop and good contacting efficiency.

It is also an object of the invention to provide a packing which islight in construction so that it can be manually shaped and cut withsimple tools if so required.

According to the invention, a packing for a vapourliquid and gas-liquidcounter-current contacting system comprises a cellular structure thecells of which have inclined walls made of single sheet foraminousmaterial of a mesh capable of supporting sheet-like liquid flowrupturable uniformly to cause bubbling or spraying by crosswise gasflow.

The preferred material of construction is expanded metal of the kindknown as mill-expanded metal sheet. It is known to produce expandedmetal in two forms; one form, which is commonly used in plasterwork isreferred to as expanded metal lath and is characterised in that itcustomarily has a large mesh and that the nodal webs in the metal are,in the expanded direction, alternately vertical and inclined to theplane of the metal. The other form, herein referred to as mill-expandedmetal sheet, and sometimes referred to as precision-cut expanded metaland commonly used for decorative grilles can be made in a smaller meshand is characterised in that the nodal Webs in the metal are allinclined to the plane of the sheet and at the same angle, the aperturesin the metal being of diamond shape.

The mill-expanded metal sheet is preferably arranged to form the packingwith the plane of the sheet inclined at between 15 and 55 and the nodalwebs approximately horizontal. The sheets can be assembled to formsimilarly sectioned horizontal prismatic cells, such as diamond shapedcells. In such an assembly the size of the cells is an optimisation ofvarious factors and a cell having a height of the order of six inchesand a width of ten inches regardless of column size is found to besuitable. With smaller cells the Height Equivalent to a TheoreticalPlate (H.E.T.P.) decreases but throughput decreases also whilst withlarge cells a gain in throughput is counterbalanced by an increasedI-I.E.T.P. figure.

In the study of the behaviour of various single sheet foraminous packingmaterials for vapour-liquid and gas liquid counter-current contactingsystems, it is observed that for some of those materials the wettedappearance is not uniform and spray formation is irregular underoperating conditions and they have a poor contacting efficiency whilstother materials acquire a more uniform wetted appearance and sprayformation and have a superior contacting eificiency. In the poormaterials it is observed that some parts appear to be filmed with liquidfrom which spray arises only at high vapour or gas rates whilst otherparts appear to be mainly dry with limited spray formation. In thebetter materials most of the packing appears to be filmed, the filminghaving a shimmering appearance which suggests a rapid and repeatedbreaking and reforming of the films over the apertures in the material,and there is copious spray formation which brings about redistributionof the liquid phase over the packing.

An analysis of the above behaviour has led to the conception that inpoor materials the ratio of resistance to gas or vapour flow between thefilmed and the dry parts of the packing is so great that the gas orvapour fiow readily by-passes the filmed parts through the dry partssuch that spray formation is inhibited and then in the absence of acopious liquid spray the dry parts do not become filmed.

In the better materials the ratio of resistance to gas or vapour flowbetween the filmed and dry parts of the packing is smaller, the filmingis more uniform and is broken and reformed uniformly due to theredistribution caused by the copious spray formation.

It is convenient now to define a ratio, called the filmed to dry ratioof a packing, as the ratio, for a given gas flow, between the resistanceto that gas flow with liquid flowing over the packing to the resistanceto the same gas flow with no fluid flowing over the packing. With thisdefinition the invention can also be stated as a packing for avapour-liquid and gas-liquid counter-current contacting systemcomprising single-sheet foraminous material shaped to provide a cellularstructure having inclined walls the mesh of the material being of such asize and shape as to have a filmed to-dry ratio in an air-water systemof less than ten when operating with an air flow of 75% that at theflooding point.

The reciprocal of the fllmed-to-dry ratio is a measure of the ease withwhich the sheet-like liquid flow in a packing is ruptured, which is afunction of the mesh size and shape. The size is required to beintermediate between the capillary openings of Stedman which seal withliquid and the non-filming openings of Scofield. The shape is preferablysuch as to require the gas to change its direction abruptly in passingthrough an opening in the packing and the liquid to cascade over theopening and only require to be deflected slightly by the gas in order tobe blown clear of the packing. These features of size and shape of themesh openings which reduce the fllmed-to-dry ratio in a packingotherwise designed to support unbroken sheet-like liquid flow, whileprimarily intended to promote the early onset of spray formation and soprevent channelling, are also, when combined with a cellular structurehaving inclined walls of single sheet foraminous material, especiallyexpanded metal, capable of producing a copious uniform spray formationover a large range of gas rates.

The invention is further described with reference to the accompanyingdrawings wherein:

FIG. 1 is a plan view of part of a sheet of mill-expanded metal sheet.

FIG. 2 is a sectional elevation on the line IIII of FIG. 1.

FIG. 3 is a sectional elevation of an inclined sheet of mill-expandedmetal sheet showing its use as a packing material for vapour-liquid andgas-liquid counter-current contacting systems.

FIG. 4 is an elevation of a sheet of one'form of the packing cut to formone wall of a diamond shaped lattice packing.

FIG. 5 shows diagrammatically the assembly diamond shaped lattice.

FIG. 6 shows the junction of four sheets of the kind shown in FIG. 4. 7

FIG. 7 shows an end elevation of flange-ended form of single sheet toform one wall of a lattice.

FIG. 8 is a sectional view showing a junction and of the 7 method .ofsecuring four sheets of the type shown in FIG. 7.

FIG. 9 is a diagrammatic elevation of part of an assembled packing usingsheets of the type shown in FIG. 7.

FIG. 10 is a sectional view of one complete cell of a packing showingone form of construction.

FIG. 11 is a perspective view of another assembled packing using adifferent form of construction.

FIG. 12 is a sectional elevation of a further form of packingconstruction.

FIG. 13 is a graph showing pressure drops for a variety of materials(each identified by a number) in the .dry state.

FIG. 14 is a graph showing pressure drops for a variety of materials(similarly identified as in FIG. 13) in the filmed state.

FIG. 15 is a graph showing the filmed to dry ratios for the materialsused in FIGS. 13 and 14.

In FIGS. 1 and 2 the mill expanded metal sheet has webs 1 connectingnodal webs 2. All these webs have a common inclination so that the websin a vertically arranged sheet are disposed in the fashion of verticalor horizontal louver boards according to the orientation of the sheeti.e. with the expanded incisions vertical or horizontal respectively.Apertures 3 exist between the webs, their larger dimension being abouttwice the smaller dimension.

In FIG. 3 a mill-expanded metal sheet 4 (like that shown in FIGS. 1 and2) is shown inclined at 45 to form one wall of a diamond-shaped lattice5. The nodal webs are seen to take up an approximately horizontalposition with a slight downward inclination. This inclination is notcritical, however, and may be inclined up to i2030 from the horizontalwithout seriously affecting the operation of the packing. Liquid flowingover such a sheet or fed to the sheet at the top passes over the surfacewith very little penetration; this flow is shown by the broken linearrows 6. The apertures in the sheet allow the passage of vapourcrosswise to the liquid flow as shown by the broken line arrows 7.

One form of assembly of the lattice is now described with reference toFIGS. 4 to 6. The lattice is formed with sheets 8 as shown in FIG. 4 cutto provide tongues 9 on the upper edge. Pairs of such sheets are placedtogether to form an inverted V with the tongues interlocking at theapex, thus forming the bottom layer 16 (FIG. 5) of the lattice.Additional layers 18a, 10!) etc. are built up on the bottom layer asshown in FIG. 5. FIG. 6 shows in more detail the junction of four sheetsof the packing. Sheets 8a and 8b having tongues 9a and 9b along theirupper edges are interlocked and sheets 8c and 8d have their lower edgesresting in the cradle formed by tongues 9a, 9b. The nodal webs 2 areshown disposed in horizontal louver fashion, gas flow through thepacking is shown by arrows 7a and liquid flow over the packing by arrows6a.

Another form of lattice assembly is now described with reference toFIGS. 7 to 9. In FIG. 7 a mill-expanded metal sheet 11 has an inclinedpart 12 and flanges 13, 14. The flanges each have holes 15 which areused to accommodate assembly bolts in a manner shown in FIG. 8. Foursheets 11 are arranged so that their flanges overlap and their holes 15coincide. The sheets are assembled so that their nodal webs 2 areapproximately horizontal and the sheets obtain support from tubularspacers 18. A bolt 17 passes through the holes 15 and tubular spacers18. In the assembly of FIG. 9 an assembled packing is shown in contactwith a container 19. The uppermost Walls and the side walls of thelattice are wired with loops 20. In use, liquid flows down the walls ofthe packing as indicated by arrows 6a.

There is convergence of liquid at the junctions 22 and followed bydivergence along the walls of. the next layer in the lattice. Gas flowthrough the lattice is shown by arrows 7a.

In FIG. 10 the cell 21 of the packing, which is of sixsided prismaticshape, is defined by four wall members 22, 23, 24 and 25 of expandedmetal which are similar in shape but orientated diiferently. Each wallmember consists of a horizontal part 26, a vertical part 27, andinclined part 28 and a horizontal part 29. The parts 26 and 29 arebolted together with bolts 30 and periodically throughout the packingscrewed spacers 31 are provided with cross-ties 32'to give rigidity andsupport to the Whole packing. At the wall side of the packing upwardlyturned lips 33 are provided on the parts 26 and 29.

In the operation of the packing liquid flows sheetwise over the members22, 23, 24 and 25- as indicated by arrows 6a (in the absence of gas orvapour flow) whilst the vapour or gas flows crosswise through thepacking as indicated by arrows 7a to rupture the fluid sheet. For fastIgas or vapour velocities a spray 36 is set up in the cell 21 which iscarried over to the adjacent wall so that in operation it appears thatthe walls of the cells are continually and uniformly wetted. V

In FIG. 11 an assembled rectangular pack is shown to form a diamondlattice cell construction. Each wall of the cells is made from a singlesheet 40 of mill-expanded metal having horizontal flanges 41 (upturnedas shown at 41a at the edge of the packing). The construction is similarto that of FIG. 10 except that there are no vertical wall parts 27 (FIG.10) to the cells. The sheets are spaced apart by spacers 42 and fixed toa frame 43. A series of such elements may be fitted into a column afterassembly outside the column, or for large columns the assembly may bebuilt up sheet by sheet inside the column. Where spacers 42 are notfitted clamping is obtained by bolts 44; alternatively small spot weldscan be made.

A method of suspending a six-sided prismatic cell packing in accordancewith the invention is shown in FIG. 12. Each cell is defined by fourwall members 22a, 23a, 24a and 25a having. top and bottom verticalextension 27a which completely overlap the vertical extension ofadjoining wall members and form 4-layer vertical walls which are securedtogether by bolts 44a. Flat strip metal hangers 17a extend verticallythrough each cell in the upper row and are sandwiched between the pairsof walls constituting the 4-layer vertical walls of the row below and soon down through the pack. They are secured by the bolts 44a. The hangersare secured top and bottom to horizontal frame members 43a and the2-layer vertical walls at the edge of the pack are secured directly tothe column walls by bolts 44b.

In another particular form of invention, 24 S.W.G. stainless steel basesheet is expanded by a mesh knife having 2 /2 serrations per inch andmeans for shifting the knife sideways by one-half of a serration betweenstrokes so that five holes per inch long-way of the mesh is obtained andthe stroke of knife is adjusted to give 4 /2 holes per inch short-way ofthe mesh with a web of one-sixteenth of an inch, and the sheet isassembled to form square-section prismatic cells having five inch sidewalls inclined at 45. Such an assembly has been found suitable in anair/water test column. Wall slopes of less than 45 may be used to obtaina thicker liquid sheet flowing over the walls but in a simple diamondcell the distance between roof and floor in each cell is thereby reducedwhich leads to earlier flooding. Wall slopes of more than 45 provide athin liquid sheet with poor efliciency at low liquid rates. With waJlslopes of less than 45 vertical side walls (walls 27 in FIG. and 27a inFIG. 12) can be introduced to increase the height of the cells so as tobe suitable for high vapour and low liquid rates.

Referring to FIGS. 13-15, the tests from which the graphs were obtainedwere carried out in a rectangular section column 2 x with the materialarranged like that shown in FIG. 11 in a 7" square diamond lattice witha water flow rate through the packing of 15 gallons per minute.

Materials No. 51, 53 and 54 are Mr" mesh millexpanded metal sheet havingfive holes per inch longway of the mesh and four and a half holes perinch short-way of the mesh. Material No. 52 is Mr" mesh gauze. Material51 is orientated so that the nodal webs are substantially horizontal asshown in FIG. 3; material 53 is orientated so that the nodal webs arevertical and material 54 is orientated so that the nodal webs areoblique; this being achieved by arranging the sheets sideways on ascompared with the arrangement for material 51, i.e. so that liquid flowis in the direction of the expanded incisions.

In FIG. 15 the limit for a filmed to dry ratio of 10 is shown as adotted line 55 and as in accordance with the invention it is seen thatmaterials 51 and 54 fulfill the condition of a filmed to dry ratio ofless than ten; the other materials far exceeding this ratio.

It is considered that the downward passage of the fluid in a packingaccording to the invention is mainly by way of rebounding spraydroplets, a droplet of liquid being blown ofi one part of the packing tobe deposited on another part Where it spreads on impact to combine withother liquid whence a further droplet is formed and the process repeateddown the packing; this is supported by the good contacting efficiencywhich suggests a copious breaking and reforming of the liquid in itspassage through the packing.

It is also considered that the low filmed-to-dry ratio possessed by thepacking prevents the onset of the phenomenon known as channelling whichis known to limit the size of column in many forms of packing to aboutthree foot.

The lattices shown in the drawings are cheaper to construct than thoselattices made from mats consisting of several layers of expanded metaland hence it becomes a reasonable economic proposition to use for theirconstruction selected high cost materials such as stainless steel orspecial purpose plastics. They are also substantially lighter thanmulti-layer packings with a consequent saving in constructional costsand reduction of internal supporting members.

I claim:

1. A packing for vapour-liquid and gas-liquid counter current contactingcolumns comprising single layer sheet material arranged to form thewalls of a stack of prismatic cells, the walls being inclined to thevertical line of the stack, the single sheet being formed of expandedmaterial with all of the nodal webs thereof disposed within an angle of30 of a horizontal position so as to provide cascading of the processliquid over the mesh openings in the form of a film, the materialextending uninterruptedly over the entire cross-section of the stack,the mesh of the material being intermediate a small size at which underoperating conditions the openings would seal with liquid under treatmentand a large size at which closure by capillary action by said liquidwould be prevented whereby under said condition the film is disrupted atany opening by the process vapour or gas to form a spray as readily asthe film is formed.

2. A packing for vapour-liquid and gas-liquid counter current contactingsystems comprising single sheet material arranged to form the walls of astack of similar prismatic cells, the walls being inclined to the lineof the stack, the single sheet being formed of mill-expanded materialwith the nodal webs thereof horizontally disposed, the materialextending uninterruptedly over the entire cross-section of the stack,the mesh of the material being intermediate a small size at which theopenings seal with liquid under treatment and a large size at whichclosure by capillary action by said liquid is prevented.

3. A packing according to claim 2 the material having a filmed-to-dryratio in an air water system of less than ten when operating with an airflow of about that at at the flooding point.

References Cited in the file of this patent UNITED STATES PATENTS2,047,444 Stedman July 14, 1936 2,227,164 Stedman Dec. 31, 19402,290,162 Bragg July 21, 1942 2,470,652 Scofield May 17, 1949 2,594,585Ridgway Apr. 29, 1952 FOREIGN PATENTS 123,245 Australia Jan. 3, 1947147,213 Great Britain Apr. 7, 1921 427,087 Great Britain Apr. 16, 1935869,527 France Nov. 7, 1941

